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still4given 15th November 2004 11:32 PM

Does physical size matter
 
I bought some Filter caps on ebay for use in the P101 power amp I am building. They are Cornell Dubilier, 15000UF 80VDC, +20% -20%, -40 C to +85C.

Part number 658-411-9838

I just received them and am a little surprised at how small they are. They measure 1 3/8"D X 2 3/8". Is there a disadvantage in using these smaller caps?

Thanks, Terry

Stuart Easson 16th November 2004 12:11 AM

specs...
 
Hi,

Bearing in mind that the density of caps has been slowly improving over the years...

Assuming they are new, you might check the manufacturers specs. There are a number of parameters that *could* be reduced to make the caps smaller, if the values are still above what you need then you can use them with no problem. Caps have a ripple current rating, and a series resistance. Both of these are important for longevity and their smoothing capability, they can only charge and discharge as fast as the ripple rating and ESR allow, if you try and use them harder they will generate some heat internally, perhaps a lot, depending on just how hard you plan to use them. There is also a temperature rating, above this temp the lifespan of the device will be compromised. You already know about maximum voltage, but your caps may not have a 'surge' rating, some caps are explicitly rated ie 20% overvoltage for 10 secs is allowed, makes for more robust equipment that is expected to get some abuse.

Hope that helps

Stuart

still4given 22nd November 2004 08:03 PM

Ok, I wrote to the supplier and these are the ratings I was given.

MAXIM. ESR .017 RIPPLE 8.80 120 HZ.

Does this sound alright for my needs?


Thanks, Terry

Stuart Easson 22nd November 2004 10:32 PM

sounds normal to me...
 
Hi,

The caps sound like they are great. The ripple rating is not high, but still corresponds to a very high continuous output level and unless your amp is to be used professionally at a dance club or such like I wouldn't worry, the ESR is normal, I think you will do fine with them.

When you parallel caps the values are generally improved in proportion to the number of caps, so 2 of those used together per pole, ie 4 total, would have an ESR of 0.0085, and ripple current of 17.6A.

Good luck

Stuart

jacco vermeulen 25th November 2004 09:26 AM

Re: specs...
 
Quote:

Originally posted by Stuart Easson
your caps may not have a 'surge' rating, some caps are explicitly rated ie 20% overvoltage for 10 secs is allowed, makes for more robust equipment that is expected to get some abuse.
Stuart

And some get lucky because of it.
I bought a large number of 78000uF mallory's from a computer guy because the caps could not handle the surge demands.
The Mallory capacitors can handle 20%, and are huge, 10"*3"
I posted a picture of one of the boxes of caps on a thread.

If possible, could you post a picture of the cap, it is indeed small, a regular one of that size would be a 40-50 volts type.

jacco


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